How do you determine run time in boats

[jotto]

How do you guys determine when it's time to swap batteries in your boat?

I'm flying RC helicopters with mAh telemetry, and I have a mAh counter alarm that tells me when it's time to land. Back in the old days I used a timer, but I guess the run times can vary quite a bit when using an electric boat (unless you're racing and running at full throttle all the time).

I have a Spektrum DX3S where I'm going to use voltage telemetry and add an alarm on the main battery voltage. It's not as good as mAh telemetry, but I think it's at least better than using a timer.

[Spartanator]

We still use timers... I also use the cheap Traxxas telemetry which has worked for so far...

[Idontswim]

With new packs, I run the boat for about 1-1:30 minutes. Bring the boat in, charge up batteries and see how much mah you put back in. Example, say your running 6000mah packs and you put back in 2100mah back in for a 1:30 run then you can set timer for 3 mins, run boat, then bring back in and check voltage. I usually run my batteries to about 3.79-3.80 volts per cell..good luck!